Ars photo essay: standing in the beam line of a neutrino detector

Join us as we travel underground at Fermilab, get bombarded by neutrinos that …

Although I was lucky enough to tour Brookhaven's RHIC accelerator during a period of scheduled downtime, my trips to the LHC and Fermilab both took place while the particle accelerators were in operation. Given the tremendous energies involved, it meant that it was simply not safe to go anywhere near the active hardware, since that's a sure way to pick up a healthy dose of ionizing radiation. But Fermilab had an exception to that, a place where it wasn't just acceptable to look at working hardware, but it was actually possible to walk right through a particle beamline. The secret? The particles were neutrinos.

Neutrinos are uncharged particles and are so light that, for decades, most physicists assumed they were actually massless. As if that weren't enough, they only interact with other matter via the weak force, which is only significant at short distances. Thus, for the most part, they generally pass through matter without incident—trillions go through your body every minute, but most of us will only have them hit anything a total of about three times in our entire lives.

They are so disinterested in interacting with matter that Fermilab is able to create a beam of neutrinos and direct them to a mine in Minnesota without losing enough of them on the way to interfere with the experiment.

Since neutrinos aren't interested in doing much other than shooting through the Universe at nearly the speed of light (given their extremely low mass, it doesn't take much energy at all to get them there), how do physicists actually work with them? That's what we've got the photos for.

Accelerating the neutrinos isn't an issue, but creating a beam of them is—since they're uncharged and not prone to much in the way of interactions, there's no way to focus them. So the people at Fermi don't. Instead, they focus the particles that decay into neutrinos. This starts by taking some of the protons out of the chain of accelerators that normally boosts them to high energy before their injection into the Tevatron. Instead, these protons are directed at a solid target, where they create a shower of unstable particles, many of which are charged.

That spray is focused into a beam using a combination of a metal horn, shown here, and a precisely timed electrical pulse.

A focusing horn, used to create a beam of neutrinos, with physicist and Minos spokesman Rob Plunkett provided for scale.

Deborah Harris, the other physicist who gave us a tour of Fermi's on-site neutrino experiments, said that the electric pulses, timed to coincide with the arrival of the particles from the solid target, are powerful enough to make the horns hum. To demonstrate, she sent along an audio file.

This isn't a perfect process—by the time the beam gets to Minnesota, it's about a kilometer in diameter—but it's good enough to send a high concentration of neutrinos in a fairly specific direction, something nature is generally not inclined to do. The showers of charged particles involved, however, makes getting close to that part of the experiment very dangerous when it's active. So we took a short drive out to the building put in place to house Fermi's Minos experiment.

The unassuming exterior of the building that provides access to the underground area that houses the Minos and Minerva detectors.

All the action goes on underground; the building is there to provide access to the site, which is a few dozen meters beneath the surface. When new hardware is put in place, it's sent down this drop shaft. The Fermi staff was kind enough to let me ride the elevator.

When a detector is too big for the elevator, a crane lowers it through here.

The elevator wasn't the only way back to the surface, however. In case of an emergency, we were warned we might be asked to walk back out the line the boring equipment had created when it carved out the underground facility.

The emergency escape route, created by the hardware that carved out the area that houses the detectors.

Once at the bottom of the shaft, Harris led us past some areas where water was dripping from the ceiling. This isn't a problem for the hardware; in fact, the water gets used to cool equipment before getting piped back to the surface.

Some parts of underground facility look like a cave, others like an office building.

Very cool! More of these types of articles please! I'm a huge fan of industrial photography -- specifically industrial decay photography -- so these are some cool pics. That photo of the escape tunnel would make an excellent wallpaper.

First thing I thought of when I read the name Minerva was the mod for Half-Life 2 so I got a good laugh from the last photo with Half-Life quotes left by the scientists. I wonder if they ever played the Minerva mod?

Since neutrinos aren't interested in doing much other than shooting through the Universe at the speed of light (given their extremely low mass, it doesn't take much energy at all to get them there), how do physicists actually work with them? That's what we've got the photos for.

That can't be right. If they have any mass at all, getting them flying at the speed of light would require infinite energy. This should be "near the speed of light". In that case, what fraction of c?

I think I've seen this place before. Are you sure this isn't Aperture Science...?

If you play through Portal 2 on developer commentary, one of the nodes talks about the sorts of reference materials they used in designing the facility. I think they mentioned underground neutrino detectors.

Since neutrinos aren't interested in doing much other than shooting through the Universe at the speed of light (given their extremely low mass, it doesn't take much energy at all to get them there), how do physicists actually work with them? That's what we've got the photos for.

That can't be right. If they have any mass at all, getting them flying at the speed of light would require infinite energy. This should be "near the speed of light". In that case, what fraction of c?

A quick look on wiki and reading one of the papers it linked to, they estimate that neutrinos travel between .999976c and 1.000126c. Also an older study using supernova 1987a found it to be between .999999998c and 1.000000002c (for higher energy neutrinos).

A quick look on wiki and reading one of the papers it linked to, they estimate that neutrinos travel between .999976c and 1.000126c. Also an older study using supernova 1987a found it to be between .999999998c and 1.000000002c (for higher energy neutrinos).

I think that's pretty damned close to saying the speed of light.

I think it might be safe to throw out those faster than the speed of light estimates. Even though they might travel extremely close to the speed of light, "near" still is not the same as "at".

But to those who say neutrinos can't move the speed of light because they have mass... there's actually a debate about whether photons themselves have mass, and it's not settled. Therefore conceivably the same argument works for neutrinos which is that they have either no or VERY FINITE rest mass, and thus can actually accelerate to the speed of light with relatively little effort, which matches our observations.

>But Fermilab had an exception to that, a place where it wasn't just acceptable to look at working hardware, but it was actually possible to walk right through a particle beamline. The secret? The particles were neutrinos.

Since neutrinos aren't interested in doing much other than shooting through the Universe at the speed of light (given their extremely low mass, it doesn't take much energy at all to get them there), how do physicists actually work with them? That's what we've got the photos for.

That can't be right. If they have any mass at all, getting them flying at the speed of light would require infinite energy. This should be "near the speed of light". In that case, what fraction of c?

Cosmological estimates limit the mass of the sum of the three neutrino masses (\nu_e, \nu_\mu and \nu_\tau) to be less than 0.3 eV (arxiv:astro-ph/0602155). Typical reactor neutrino energies are ~1 GeV -- this is a low energy for a modern accelerator, but high compared to perhaps more familiar neutrinos -- those coming from the PP1 fusion chain in the sun have energies of ~1 MeV. Let's do the maths with both. So, let's do some relativity, kids: say a neutrino is 0.1 eV, then E=\gamma mc^2; taking E=1 GeV and m=0.1 eV = 1*10^-10 GeV gives \gamma = 10^10. For a ~1 MeV neutrino, \gamma is 10^7.

Now then, \gamma=1/sqrt(1-v^2/c^2)=(1-\beta^2)^(-1/2). So then, rearranging gives 1-\beta of 5*10^21, in the case of the 1 GeV neutrino, and 5*10^-15 in the case of the 1 MeV neutrino.

In other words, they're going at least 99.9999999999999999995000000000% or 99.9999999999994999999999999987% of the speed of light, respectively for the "high" and "low" energy neutrinos.

They're going at c, and their mass is, to first order, able to be neglected :-).

Neutrinos, they are very small.They have no charge and have no massAnd do not interact at all.The earth is just a silly ballTo them, through which they simply pass,Like dustmaids down a drafty hallOr photons through a sheet of glass.They snub the most exquisite gas,Ignore the most substantial wall,Cold shoulder steel and sounding brass,Insult the stallion in his stall,And, scorning barriers of class,Infiltrate you and me. Like tallAnd painless guillotines they fallDown through our heads into the grass.At night, they enter at NepalAnd pierce the lover and his lassFrom underneath the bed—you callIt wonderful; I call it crass.—John Updike

Neutrinos, they are very small.They have no charge and have no massAnd do not interact at all.The earth is just a silly ballTo them, through which they simply pass,Like dustmaids down a drafty hallOr photons through a sheet of glass.They snub the most exquisite gas,Ignore the most substantial wall,Cold shoulder steel and sounding brass,Insult the stallion in his stall,And, scorning barriers of class,Infiltrate you and me. Like tallAnd painless guillotines they fallDown through our heads into the grass.At night, they enter at NepalAnd pierce the lover and his lassFrom underneath the bed—you callIt wonderful; I call it crass.—John Updike

When I was a sprite of 15 years, my physicist father and I went to visit a "neutrino telescope" in/under London when we spent a year there (he as a visiting professor at Queen's College, London and I as student at Dulwich College, alma mater of P.G.Wodehouse). The facility consisted of a tank of a million gallons or so of water for the detector, and they were using the earth's core as a "lens"... Neutrino physics is a very esoteric field! :-) FWIW, my wife is a staff physicist at Fermi Lab.

When you see what lengths must be taken to detect a few neutrinos, it really makes the Star Trek tricorders which can detect not only neutrinos but tachyons and all sorts of other particles amazing devices.

I had a physics class at the University of California, Irvine some years back and after the first few classes, I had a question and stayed after class to ask the professor for help and he said to come with him to his office and he would explain it to me. We talked along the way and he later answered my question in his office.

Some time later, I somehow found out that the professor was one of the discovers of the neutrino, Frederick Reines. I had no idea at the time that I was talking to and being helped by such am esteemed physicist.

As far as I understood it from my particle physics courses, neutrinos can move faster then light of a specific wavelength in a given material, since they don't interact with the matter yet light does. This gives rise to the Cherenkov-radiation used to detect neutrions in the first place.So in some way they really are moving at the speed of light or even above it. Just not faster then the vacuum speed of light.

Not quite. If you can bump up the temperature of a lump of iron of about 1.44 solar masses to about 100 billion Kelvin (by, say, exceeding its electron degeneracy pressure leading to sudden collapse) then you can turn lots of gravitational potential energy into thermal energy into lots of neutrinos, enough to blow up a rather large (> 10 solar mass) star, which is a pretty nifty (if unfocussed) death ray.

Tellur wrote:

As far as I understood it from my particle physics courses, neutrinos can move faster then light of a specific wavelength in a given material, since they don't interact with the matter yet light does. This gives rise to the Cherenkov-radiation used to detect neutrions in the first place.

Cherenkov radiation is only produced by charged particles, so some neutrino detectors work by seeing the flashes caused by the particles that they bump into and kick around, not the neutrinos themselves.

I had a physics class at the University of California, Irvine some years back and after the first few classes, I had a question and stayed after class to ask the professor for help and he said to come with him to his office and he would explain it to me. We talked along the way and he later answered my question in his office.

Some time later, I somehow found out that the professor was one of the discovers of the neutrino, Frederick Reines. I had no idea at the time that I was talking to and being helped by such am esteemed physicist.